This invention relates generally to subterranean boreholes, and in particular to systems for controlling the operating pressures within subterranean boreholes.
Referring to FIG. 1, a typical oil or gas well 10 includes a wellbore 12 that traverses a subterranean formation 14 and includes a wellbore casing 16. During operation of the well 10, a drill pipe 18 may be positioned within the wellbore 12 in order to inject fluids such as, for example, drilling mud into the wellbore. As will be recognized by persons having ordinary skill in the art, the end of the drill pipe 18 may include a drill bit and the injected drilling mud may used to cool the drill bit and remove particles drilled away by the drill bit. A mud tank 20 containing a supply of drilling mud may be operably coupled to a mud pump 22 for injecting the drilling mud into the drill pipe 18. The annulus 24 between the wellbore casing 16 and the drill pipe 18 may be sealed in a conventional manner using, for example, a rotary seal 26. In order to control the operating pressures within the well 10 such as, for example, within acceptable ranges, a choke 28 may be operably coupled to the annulus 24 between the wellbore casing 16 and the drill pipe 18 in order to controllably bleed off pressurized fluidic materials out of the annulus 24 back into the mud tank 20 to thereby create back pressure within the wellbore 12. The choke 28 is manually controlled by a human operator 30 to maintain one or more of the following operating pressures within the well 10 within acceptable ranges: (1) the operating pressure within the annulus 24 between the wellbore casing 16 and the drill pipe 18xe2x80x94commonly referred to as the casing pressure (CSP); (2) the operating pressure within the drill pipe 18xe2x80x94commonly referred to as the drill pipe pressure (DPP); and (3) the operating pressure within the bottom of the wellbore 12xe2x80x94commonly referred to as the bottom hole pressure (BHP). In order to facilitate the manual human control 30 of the CSP, the DPP, and the BHP, sensors, 32a, 32b, and 32c, respectively, may be positioned within the well 10 that provide signals representative of the actual values for CSP, DPP, and/or BHP for display on a conventional display panel 34. Typically, the sensors, 32a and 32b, for sensing the CSP and DPP, respectively, are positioned within the annulus 24 and drill pipe 18, respectively, adjacent to a surface location. The operator 30 may visually observe one of the more operating pressures, CSP, DPP, and/or BHP, using the display panel 34 and attempt to manually maintain the operating pressures within predetermined acceptable limits by manually adjusting the choke 28. If the CSP, DPP, and/or the BHP are not maintained within acceptable ranges then an underground blowout can occur thereby potentially damaging the production zones within the subterranean formation 14. The manual operator control 30 of the CSP, DPP, and/or the BHP is imprecise, unreliable, and unpredictable. As a result, underground blowouts occur thereby diminishing the commercial value of many oil and gas wells.
The present invention is directed to overcoming one or more of the limitations of existing systems for controlling the operating pressures of subterranean boreholes.
According to an embodiment of the present invention, a method of controlling one or more operating pressures within a subterranean borehole that includes a tubular member positioned within the borehole that defines an annulus between the tubular member and the borehole, a sealing member for sealing the annulus between the tubular member and the borehole, a pump for pumping fluidic materials into the tubular member, and an automatic choke for controllably releasing fluidic materials out of the annulus between the tubular member and the borehole is provided that includes sensing an operating pressure within the tubular member and generating an actual tubular member pressure signal representative of the actual operating pressure within the tubular member, comparing the actual tubular member pressure signal with a target tubular member pressure signal representative of a target operating pressure within the tubular member and generating an error signal representative of the difference between the actual tubular member pressure signal and the target tubular member pressure signal, and processing the error signal to generate a set point pressure signal for controlling the operation of the automatic choke.
The present embodiments of the invention provide a number of advantages. For example, the ability to control the DPP also permits control of the BHP. Furthermore, the use of a PID controller having lag compensation and/or feedforward control enhances the operational capabilities and accuracy of the control system. In addition, the monitoring of the system transient response and modeling the overall transfer function of the system permits the operation of the PID controller to be further adjusted to respond to perturbations in the system. Finally, the determination of convergence, divergence, or steady state offset between the overall transfer function of the system and the controlled variables permits further adjustment of the PID controller to permit enhanced control system response characteristics.